The prior art teaches electric machines, such as switched reluctance motors, which operate on the basis of varying reluctance in their several magnetic circuits. Typically, such machines are doubly salient, that is, each machine has poles on both its stator and its rotor. The stator poles are typically encompassed by electric windings to form the phases of the machine and, in a common configuration, the stator windings on diametrically opposite poles are connected in series to form one machine phase. The rotor's poles are likewise disposed about the rotor's periphery in diametrical pairs.
When a stator phase is energized, the resulting magnetic field tends to urge the nearest rotor pole pair toward an aligned position with the poles of the energized stator phase, thereby applying a torque to the rotor whose profile is itself dependent upon several factors, including the shapes of the poles and their relative rotational positions, and the manner in which the stator phase is itself energized, e.g., the current applied to the stator windings. By energizing consecutive stator phases in succession, and in a cyclical fashion, the rotor may be urged to rotate continuously by virtue of the resulting overlapping torque profiles.
Unfortunately, the nominal torque profile for each stator phase is such that the overlapping torque profiles themselves combine to produce a varying overall torque output for the machine. In response, the prior art teaches varying the amplitude of the electric current applied to each stator winding as a function of the relative angular position of the rotor (sometimes referred to as "current mapping"), thereby generating machine phase torque profiles which, when combined, provide a nearly constant machine torque output. This prior art approach, however, requires additional electronic circuitry with which to sense the relative angular position of the rotor and to modulate the applied current in response thereto. Such additional circuitry greatly increases the cost and complexity of the resulting machine.
Thus, there is a need to provide an electrical machine that reduces or eliminates one or more problems as described above.